The Nuclear Weapons Complex Management for Health, Safety, and the Environment (1989) / Chapter Skim
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Appendix E: Physics of Nuclear Weapons Design
Appendix E: Physics of Nuclear Weapons Design
Pages 123-128
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From page 123... ...
Before it is fired, the fissile material in the weapon must be in a subcritical configuration, and it will require some time to move the material to make the highly supercritical assembly. The final part of this time—after the material has first become critical, but before it has reached the desired fully assembled stateis the "supercritical time," during which the fissile material is capable of sustaining a chain reaction.
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From page 124... ...
Because the rates for alpha decay of the plutonium isotopes are very much higher than for uranium, the alpha-e source in weapons-grade plutonium is about a 1, 000 times larger than that in highly enriched uranium at the same purification level. The difference does not matter, however, because the neutron source from spontaneous fission in weapons-grade plutonium is larger than that in highly enriched uranium by a factor of more than 10,000.
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From page 125... ...
When the explosive is detonated on its outer surface in such a way that a spherically converging shock wave is imposed on the fissile material compressing it by a factor between about four thirds and two, the assembly is highly supercritical by the time the shock wave reaches the center. With this method of assembly, the supercntical time may be only a few microseconds, as compared with the few hundred microseconds required in the case of the gun method.
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From page 126... ...
Being introduced quite independently of the progress of the chain reaction, and in a near-instan~neous pulse, the neutrons increase the rate of fissions very sharply, with the result that the yield ultimately realized may be several-fold larger than it would have been without the "boosting." As a consequence of employing this technique, it has been possible to obtain a larger yield from a device of a given size and cost in fissile matenal, to obtain the same yield from a smaller amount of fissile material, and most importantly to obtain a desired yield from devices reduced ilk size and weight. Almost all weapons produced since about 1960 have been boosted.
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From page 127... ...
· The polonium-beryllium modulated initiator previously referred to in connection with a pure fission device had a short shelf life because of the 138-day half-life of polonium-210, and the replacement of Hose was once a major activity at the Mound facility. In boosted weapons the initiation function has been taken over by electrically powered neutron generators obtained from the Pinellas Plant.
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From page 128... ...
Such conditions are favorable for a rapid thermonuclear burning of the Li6D; and the energy from this, along with that from the fissions induced in the wall, determines the yield of the weapon. Apart from calculating the progress of processes just referred to, a main problem for the designer will be to conform to the shape and dimensional constraints imposed by the characteristics of the delivery vehicle in question, while at the same time striving to meet the conflicting desires of the military customer that the weight be reduced and the yield increased as much as possible.
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